A mixed solvent of cyclic carbonate and chain carbonate is used as the non-aqueous organic solvent in the current liquid lithium-ion battery electrolytes; both the cyclic carbonate and the chain carbonate are aprotic solvents with high viscosity and large surface tension, causing small affinity between the electrolyte and the separator material (polyethylene or polypropylene in general), and this separator material is hardly wetted by the electrolyte so as to lead to poor wettability of the electrolyte for the separator, as a result, a large contact resistance is created between the separator and the electrolyte. In addition, the electrolyte is also poor in wettability for electrode materials (i.e. positive and negative electrode materials) to create a large contact resistance between the electrolyte and the electrode material, which accordingly affects the utilization rate of the positive and negative electrode materials and is unfavorable for the functioning of battery capacity.
There are a large number of patent applications (e.g. those with the publication numbers of CN1210833, CN102544582, EP2421081 and CN101465212) in which the technical scheme below has been reported: the wettability of electrolyte is improved by adding nonionic surfactants as wettability additives, such as alkylphenol polyoxyethylene ether, long-chain aliphatic alcohol polyoxyethylene ether, aliphatic acid polyoxyethylene ether, fluorinated alkyl polyoxyethylene ether and silane compounds, etc. In addition, in the patent application CN100442592 for Samsung SDI CO., LTD., the applicant further discloses a lithium ion battery electrolyte containing trialkyl phosphates (nonionic surfactants) and dialkyl carbonates (nonionic surfactants), and the lithium ion battery using the electrolyte has excellent cycling performance and high-temperature stability. The wetting effects of the various aforementioned nonionic surfactants in the electrolyte are bridged together mainly by formation of different intermolecular hydrogen bonding between hydrophobic alkyl and hydrophilic hydroxyl, ether bonds or ester groups in the structure, and the solvent and the separator material (polyethylene, polypropylene, etc.) in the electrolyte, respectively. However, a hydrophilic group is composed of hydroxyls, ether bonds or ester groups, so in order to give molecules with sufficient hydrophilicity to further achieve good wetting effect, the number of hydroxyls, ether bonds or ester groups must be increased, and this will increase the molecular weight of the additive and the viscosity of the electrolyte. Wetting is a dynamic process, and under the same conditions, smaller molecular weight leads to faster wetting, as a result, the wetting effect becomes worse as the molecular weights of the above nonionic surfactants increase. Furthermore, polar group moieties in these nonionic surfactants are all common hydrocarbon structures or carbon-fluorine bonds, carbon-silicon bonds and silicon-oxygen bonds, and tend to have a considerable length, thereby causing worse chemical stability.
In the case that compounds containing a plurality of carbon-oxygen bonds (carbon-oxygen single bonds and carbon-oxygen double bonds) are screened, simple ether, aldehyde, ketone or alcohol compounds may all have wettability, but among the substances listed above, ethers are likely to undergo side reaction in cells, i.e. chain scission, to affect the cycling performance and other performances of the cells; and oxidation of aldehydes, alcohols and common ketones occurs quite easily, so they are all inapplicable for use as the electrolyte additives.
The technical scheme below has been reported in the patent applications with the publication numbers of CN101048912 and CN101702446 patent applications: furanone compounds are used as the electrolyte additives to improve the high-temperature storage performance of cells. These patent applications indicate that furanone compounds are capable of inhibiting electrolyte decomposition and are involved in formation of positive/negative electrode SEI films, and moreover, they do not affect the performances of adhesive materials and do not react with electrode materials or discharging products to cause gas generation during use. However, hydrogen atoms (α-hydrogen) are present on the carbonyl-connected carbon atoms in their molecules, so these compounds are liable to oxidation-reduction to result in unsatisfactory chemical and electrochemical stability.
In addition, since lithium is an alkali metal, it has the characteristics of an oxyphile element; plenty of documents have reported use of crown ether compounds for improving the degree of dissociation and transference number of lithium ions to further improve battery capacity, however, those crown ether compounds, on the one hand, are high in price, and on the other hand, their electrochemical stability is poor, which finally degrades the electrochemical properties of cells seriously.